In a standard cement manufacturing process, the raw materials needed to produce cement are first mixed and fed to raw grinding mills to produce “raw meal”. The raw meal is ground and dried in the mill and then sent to a kiln in which it is heated to about 1,500° C. to produce clinker, the basic material required for the production of all cements. The clinker is thereafter transported to a cooler and, after cooling, the material is subject to finish grinding.
In the clinker cooler the clinker is cooled from about 1,400° C. to about 90° C. by ambient air supplied by a forced draft or natural draft supply system that passes through the clinker. A predetermined portion, generally about 40-50%, of the air used to cool the clinker is thereafter directed into the rotary kiln for use as combustion air. Since a significant amount of air passing through the cooler is not utilized in the kiln, excess air from the cooler is typically vented to the atmosphere, used for drying coal or raw materials, or used as an air source in other areas of a cement plant.
A raw grinding mill is generally operated according to the principle of drying and grinding, and uses heated gases in the drying stage that are typically delivered to it from other areas of the plant, such as from the preheater and, at times, the clinker cooler.
The cement industry in the United States has a long-standing commitment to reduce the industry's environmental footprint. In the 1990s, in response to concerns about smog and acid rain, the industry installed new technologies such as low NOx burners to reduce emissions of NOx and fine particulate. Most cement plants now have emissions monitoring systems in place.
The raw grinding operation traditionally has not been a point of focus in the industry's battle against undesirable emissions, as the raw mill is simply not a major source of emissions within a cement plant. However, as government emission standards are becoming more and more stringent, the industry is looking to reduce emissions in areas that heretofore have not been under scrutiny. On that basis, it has been discovered that the grinding of raw materials can produce certain undesirable emissions, including volatile organic compounds (VOCs), methane, and CO, among others. It would be desirable, therefore, to have a method for reducing emissions generated by the raw material grinding process.